Our research group has been working on the mechanism of transport of amphipathic lipids in tissues. During the past 6 years we have developed a model for transport of fatty acids between capillary lumen and the interior of parenchymal cells by lateral movement in an interfacial continuum of the outer leaflets of cell membranes. Our studies in several tissues have shown that lamellar structures, resembling myelin figures, develop in incubated-fixed tissues under conditions causing lipolysis and accumulation of fatty acids in the tissues. We have concluded that the lamellar structures are fatty acids, products of lipolysis, which form extensions of the continuum when fatty acids overcrowd the continuum, and that the lamellae mark the course of the continuum through the tissue. Our recent studies in brown adipose tissue using the freeze fracture technique showed that fractured surfaces of lamellar structures in incubated-fixed tissue were free of intramembranous particles, and that in some areas the fractured surfaces were continuous with those of the external leaflet of cell membranes. These findings support our concept that lipolytic products travel in the external leaflets of membranes, and that they can form lamellar extensions of the leaflets under certain conditions. We have observed formation of myelin figures from lipolytic products perifused in a flow-through chamber with buffer solution at pH 8.1. Myelin figures with large aqueous spaces often projected from lipolyzed droplets as tubules, or as streams of interconnected vesicles. Transformation between tubules and vesicles also occurred. Sometimes myelin figures were carried by the current of the perifusate, and the long filaments connecting them with lipolyzed droplets became invisible with the light microscope. However when they were perifused with acidic solution (pH 5.5), the myelin figures returned to the loci of lipolysis as they transformed to lenses of protonated fatty acids. The rapid retraction of myelin figures when the aqueous phase was acified demonstrates the integrity of the interfacial continuum formed by lipolytic products at pH 8.1. Chemical analysis of the perifusate confirmed that very little, if any, of the lipolytic products dissolved in the aqueous phase.